I am designing a circuit with some expensive components ($60.0 plus) and I wanted to protect them from voltage spikes with Zener diode. So I took this Vishay BZX55C4V3 Zener Diode and connected it to 120 Ohm resistor and connected it to a power supply and oscilloscope.

First, as I understand, this Zener is supposed to keep voltage across itself fixed at 4.3V. But if I rump up the power supply to 10 or 11V I can push Zener to about 4.8V. So, no protection there.

Second, I have a neon lamp (one with magnifying glass) directly above my desk and above the circuit itself. Maybe 10"(25cm). If I turn on the lamp, I can see on the oscilloscope that voltage spikes up, across the diode itself, to 5.6V. Granted, this can be induction in the oscilloscope cable. Again, no protection from inductive spikes.

So what it is wrong here? It seems that these Zener diodes are neither regulating voltage from power supply, neither protecting from EMF in the environment.

How can I properly protect the circuit from voltage spikes? Is there a rock solid way to get specified voltage protection?

The simplest way to protect an input against overvoltage is a simple series resistor. The best value for the resistor depends on what voltage you need to protect against, and the function of the input.

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OK, but is there a way to just 100% solid limit the voltage to a desired value? So whenever a spike happens, the component just shaves it off clean.

I mean, all the tutorials on the web are talking about Zener diodes regulating voltage. When in reality they are quite loose. On this one that was rated 4.3V, it can go all the way to 5.0V and even higher.

OK, but is there a way to just 100% solid limit the voltage to a desired value? So whenever a spike happens, the component just shaves it off clean.

Yes, use a silicon diode or Schottky diode between the signal and a power supply with a voltage just below the desired limit voltage, with a capacitor across the supply to absorb the energy. That is how the pin protection diodes work.

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Yes, use a silicon diode or Schottky diode between the signal and a power supply with a voltage just below the desired limit voltage, with a capacitor across the supply to absorb the energy. That is how the pin protection diodes work.

Many thanks. I'll try that. I've just tried the above lamp trick on L78M09CV 9.0V Voltage Regulator and it didn't glitch at all. No spikes whatsoever. No wander one always see these things on all the Arduino breakout boards ;-). So that's the way to go. Maybe I'll use Zeners for something else.

What is really the real use for Zener diodes, if they are quite loose as voltage regulators?

As you have discovered it is a wast of time doing that. Even proper shielded cable doesn't act like you expect.What is wrong is your expectations and supposed requirements for protection.There is a balance between low impedance and effective suppression. The better the suppression the higher is the input impedance.The ultimate is to add a buffer on the front end, rather than just putting a signal into an expensive chip.

Well you haven't said what the signal or the component is but if it is a logic level signal then something like a 74LS04 or even a simple transistor will isolate anything picked up on the line from the chip.

but is there a way to just 100% solid limit the voltage to a desired value? So whenever a spike happens, the component just shaves it off clean.

Depends on your definition of "clean". Most diodes or zvs or zeners allow some degree of variations above their rated levels, more or less.

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I mean, all the tutorials on the web are talking about Zener diodes regulating voltage. When in reality they are quite loose. On this one that was rated 4.3V, it can go all the way to 5.0V and even higher.

That's how they are.

A "cleaner" solution would be a voltage reference, like tl431. However, those devices have limited ability to absorb shocks (fast / large transients).

I mean, all the tutorials on the web are talking about Zener diodes regulating voltage. When in reality they are quite loose. On this one that was rated 4.3V, it can go all the way to 5.0V and even higher.

I had a similar issue and found that zeners require a certain minimum current flowing thru them to function as a voltage limiter (check the data sheet for your zener). You might try six .7v voltage drop diodes in series to limit the board voltage to 4.2v.

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